Instruction manual

Configuration Page 3-5
Eagle 450 Installation and Technical Manual, Rev. 00
Wide Bus Termination
When terminating the Wide SCSI bus, keep two things in mind:
•
All 16 lines of the bus must be terminated.
•
Termination must be active.
If you have only internal SCSI devices, termination is simple: just make sure the external active
terminator supplied with the Eagle 450 is installed properly. Similarly, if you have only Wide external
devices, simply remove the terminator from the external port on the chassis and place it in the unused
connector on the last device on the external cable.
When you have a narrow external SCSI device, such as a CD-ROM drive, termination becomes slightly
more complicated. As stated above, any narrow external device must be the last device on the cable,
beyond any Wide external devices. You must terminate the “high” nine lines of the Wide bus before the
first external narrow device, and the rest of the bus signals at the last device. To do this:
1. Between the last Wide device and the first narrow device (if you have only narrow external
devices, between the external Wide connector and the first narrow device), you must use a Wide-
to-narrow cable which actively terminates the high nine lines. Alpha Micro offers two such
cables: PDB-00440-80 (3 ft.) and PDB-00440-81 (6 ft.). You cannot use the six-foot cable unless
you are using the AM-441Wide SCSI Bus Repeater.
2. Plug an active narrow terminator into the unused SCSI connector on the last narrow device. One
is available from Alpha Micro, part number PRA-00222-21.
In this configuration, the external Wide terminator included with the Eagle 450 is not used.
Narrow SCSI Configuration Rules
The narrow SCSI bus allows a cable length of up to 6 meters (approximately 20 feet). This means that,
unlike the Wide bus, total cable length is rarely a consideration. Instead, the limiting factors are the
number of connectors on the internal cable and the allowed number of device IDs.
The narrow bus only allows IDs from 0 through 7. Since ID 7 is used by the host controller,
you cannot
have more than seven devices on the narrow bus
.
The internal SCSI cable shipped with your Eagle 450 has only four drive connectors, so you cannot have
more than four internal devices. You can, however, easily have four internal devices and three external
ones. The external devices can be “stand-alone” devices like the AM-642 SCSI to Pertec converter, or
they can be in an expansion chassis such as the AM-3501.
Wide Devices on the Narrow Bus
You can attach any Wide SCSI device—disk drive or tape drive— to the narrow bus by using a 68-pin to
50-pin adapter, PDB-00440-90, between the device and the cable connector. However, on the narrow
bus, Wide devices do not perform any better than comparable narrow devices. By using the narrow bus
you sacrifice any performance advantage.